Typically, p-xylene is derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C.sub.8 aromatics in such mixtures and their properties are:
______________________________________ Freezing Boiling Density Lbs./ Point .degree.F. Point .degree.F. U.S. Gal. ______________________________________ Ethylbenzene -139.0 277.0 7.26 P-xylene 55.9 281.0 7.21 M-xylene -54.2 282.4 7.23 O-xylene -13.3 292.0 7.37 ______________________________________
Principal sources are catalytically reformed nahthas and pyrolysis distillates. The C.sub.8 aromatic fractions from these sources vary quite widely in composition but will usually be in the range 10 to 32 wt. % ethylbenzene with the balance, xylenes, being divided approximately 50 wt. % meta, and 25 wt. % each of para and ortho.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation although this is a costly operation. Ortho xylene may be separated by fractional distillation and is so produced commercially. Para xylene is separated from the mixed isomers by fractional crystallization.
As commercial use of para and ortho xylene has increased there has been interest in isomerizing the other C.sub.8 aromatics toward an equilibrium mix and thus increasing yields of the desired xylenes, as by OCTAFINING.
In a typical plant for utilization of Octafining, a mixture of C.sub.8 aromatics is introduced to an ethylbenzene tower wherein the stream is stripped of a portion of its ethylbenzene content, to an extent consistent with retaining all the xylenes in the feed stream without unduly expensive "superfractionation". Ethylbenzene is taken overhead while a bottom stream, consisting principally of xylenes, together with a significant amount of ethylbenzene, passes to a xylene splitter column. The bottoms from the xylene splitter constituted by o-xylene and C.sub.9 aromatics passes to the o-xylene tower from which o-xylene is taken overhead and heavy ends are removed. The overhead from the xylene splitter column is transferred to conventional crystallization separation. The crystallizer operates in the manner described in Machell et al., U.S. Pat. No. 3,662,013 dated May 9, 1972.
Because it's melting point is much higher than that of the other C.sub.8 aromatics, p-xylene is readily separated in the crystallizer after refrigeration of the stream and a xylene mixture lean in p-xylene is transferred to an isomerization unit. The isomerization charge passes through a heater, is admixed with hydrogen and the mixture is introduced to the isomerizer.
Isomerized product from the isomerizer is cooled and passed to a high pressure separator from which separated hydrogen can by recycled in the process. The liquid product of the isomerization passes to a stripper from which light ends are passed overhead. The remaining liquid product constituted by C.sub.8 +hydrocarbons is recycled in the system to the inlet of the xylene splitter.
It will be seen that the system is adapted to produce quantities of p-xylene from a mixed C.sub.8 aromatic feed containing all of the xylene isomers plus ethylbenzene. The key to efficient operation for that purpose is in the isomerizer which takes crystallizer effluent lean in p-xylene and converts the other xylene isomers in part to p-xylene for further recovery at the crystallizer.
Among the xylene isomerization processes available in the art, Octafining was originally unique in its ability to convert ethylbenzene. Other xylene isomerization processes have required extremely expensive fractionation to separate that component of C.sub.8 aromatic fractions. As will be seen from the table of properties above, the boiling point of ethylbenzene is very close to those of p and m-xylene. Complete removal of ethylbenzene from the charge is impractical. The usual expedient for coping with the problem was an ethylbenzene separation column in the isomerizer-separator loop when using catalyst other than those characteristic of Octafining. It will be seen that Octafining does not need this expensive auxiliary to prevent build up of ethylbenzene in the loop. This advantageous feature is possible because the Octafining catalyst converts ethylbenzene.
In Octafining, ethylbenzene reacts through ethyl cyclohexane to dimethyl cyclohexanes which in turn equilibrate to xylenes. Competing reactions are disporportionation of ethylbenzene to benzene and diethylbenzene, hydrocracking of ethylbenzene to ethane and benzene and hydrocracking of alkyl cyclohexanes.
A significant improvement arose with the introduction of catalysts such as zeolite ZSM-5 combined with a metal such as platinum as described in Morrison U.S. Pat. No. 3,856,872. At temperatures around 700.degree.-800.degree. F., ethylbenzene is converted by disporportionation over this catalyst to benzene and diethylbenzene. At higher temperatures and using a zeolite ZSM-5 catalyst of reduced activity, ethylbenzene and other single ring aromatics are converted by splitting off side chains of two or more carbon atoms as described in copending application Ser. No. 914,645, filed June 12, 1978.
These developments permit upgrading of Octafining reactors by substitution of the improved (ZSM-5) catalyst.
Another xylene isomerization system which has achieved widespread commercial use in low pressure operation in vapor phase. Temperatures employed are in the same range as for Octafining, in the neighborhood of 850.degree. F. Pressures are only that required to equal pressure drop through the downstream recovery towers, heat exchanges and the like. For all practical purposes, this is an atmospheric pressure reaction with reactor inlet pressure of about 30 pounds per square inch, gauge. The catalyst is essentially silica-alumina, the acid amorpohous heterogeneous catalyst employed in a number of such acid catalyzed processes. Several advantages for that type of isomerization will be immediately apparent.
The unit cost of catalyst is drastically reduced by omission of platinum. At these low pressures, the reactor vessels are made of inexpensive steel and need no structural provision for resisting pressure stress. The process is practical without introduction of molecular hydrogen and needs no auxiliaries for manufacture and recycle of that gas. These features greatly reduce capital and operating costs and have made the low pressure process essentially competitive with Octafining despite the requirement for large vessels at low pressure and low space velocity and the operating disadvantages inherent in the process.
A primary drawback of low pressure vapor phase isomerization as practiced heretofore is its low tolerance for ethylbenzene in the charge. The catalyst will convert ethylbenzene only at high severities such that unacceptable loss of xylene occurs by disproportionation.
Low pressure isomerization as practiced heretofore accepts a further disadvantage in that the catalyst rapidly declines in activity due to deposition of "coke", a carbonaceous layer masking the active sites of the porous silica-alumina catalyst presently conventional in this operation. The coke can be removed by burning with air to regenerate the activity of the catalyst. Continuity of operation is achieved by the well-known "swing reactor" technique employing two or more reactors, one of which is on stream while burning regeneration is conducted on a reactor containing spent catalyst which has lost activity by coke deposition. Cycles of two to four days are common practice using one reactor on stream for that period and then shifting to a freshly regenerated vessel.
Present commercial practice involves many large plants of both the Octafining and low pressure types in a loop of p-xylene separation and recycle of other isomers, together with such quantity of ethylbenzene as may be present, through isomerization and back to p-xylene recovery. The commercial options presently in use are high pressure isomerization with large quantities of hydrogen or low pressure (essentially atmospheric) isomerization with complicated cycling of a swing reactor and necessity for expensive distillation to remove ethylbenzene from the charge to some acceptable level, usually about 5%.
Substitution of zeolite ZSM-5 for the silica-alumina catalyst of low pressure isomerization results in an advantageous shift in the relative rates of disproportionation of ethylbenzene and xylenes. The so revised process as described in U.S. Pat. No. 4,101,596 is operated at lower temperature than conventional low pressure isomerization over silica-alumina. At these conditions, long on-stream periods are observed even at high levels of ethylbenzene in the charge permitting operation of the ethylbenzene tower at moderate, and less expensive, conditions to remove only a portion of the ethylbenzene in the charge. The so improved low pressure with zeolite ZSM-5 requires modification of the conventional low pressure plant to accommodate the lower temperature of operation.
A further alternative heretofore described is isomerization in liquid phase at a pressure adequate to maintain that phase. Highly active zeolite catalysts are effective under these conditions and demonstrate long cycle life, possibly because precursors of coke are dissolved by the reactant liquid and flushed from the reactor before deterioration to coke. See, for example, Wise, U.S. Pat. No. 3,377,400; Bowes et al., U.S. Pat. No. 3,578,723; and Haag et al., U.S. Pat. No. 3,856,871.
It is further known that zeolite ZSM-5 is a very effective catalyst for isomerization of xylenes. See Argauer et al., U.S. Pat. No. 3,790,471; Burress, U.S. Pat. No. 3,856,873; Morrison, U.S. Pat. No. 3,856,872; and Haag et al., supra. It should be noted that Burress describes a wide range of operating conditions and demonstrates effectiveness of the catalyst at (1) low temperature, high pressure and (2) high temperature, low pressure operation over zeolite ZSM-5. See also Bonacci et al. U.S. Pat. No. 3,957,621. On this state of the art, zeolite ZSM-5 can be expected to function effectively in low pressure, vapor phase isomerization, and indeed it does. That zeolite and the related zeolites are defined hereinafter by silica/alumina ratio, constraint index and crystal density.